There are many applications where a very low bit rate digitized voice signal is useful. For example, any communication system having a limited bandwidth can implement more voice channels within the bandwidth if the voice data rate is reduced. Examples of such communication systems include, but are not limited to, cellular telephone systems and satellite communications systems, such as those that employ L band communications. In general, any satellite communication scheme can employ bit reduction techniques to simplify the processing of the signals.
A primary example of the use of low bit rate voice signals is the enciphered telephone system used by the military and intelligence communities. One conventional approach for maintaining privacy on telephone uses a 16 kbit/s continuously variable slope delta modulation scheme (CVSD) in the transmission of the voice signals. However, the quality of the voice is notoriously poor, and would most likely not be used were it not for the sensitive nature of the conversations. When the bit rate is expanded to 32 kbits per second, the quality of the CVSD voice is quite good, but the data rate is large enough to consume considerably more communication bandwidth than the usual telephone channel. By comparison, a standard digital telephone channel uses 64 kbits per second.
Another known technique that is used to achieve low bit rates is linear predictive coding (LPC). Linear predictive coding achieves bit rates of 2.4 kbits per second for poor quality, but intelligible speech. However, it is often impossible to recognize the speaker when using the LPC speech.
Furthermore, LPC exhibits a problem when a noise signal coexists with the desired voice signal, in that the prediction algorithm adapts to the noise as well as to the speech. The result is that, for low signal-to-noise ratios, the speech signal may nearly disappear. This is because the noise signal "captures" the Linear Predictive Coder, and any residual of the voice signal is greatly reduced in amplitude and quality. LPC furthermore .has difficulty with both white noise and with coherent noise. Examples of coherent noise are 60 Hz hum and the hum of machinery.
The following U.S. Patents all disclose various aspects of Linear Predictive Coding (LPC) as applied to speech: U.S. Pat. No. 4,718,087, entitled "Method and System Encoding Digital Speech Information", by Panagiotis E. Papamichalis; U.S. Pat. No. 4,720,861, entitled "Digital Speech Coding Circuit", by John P. Bertrand; U.S. Pat. No. 4,724,535, entitled "Low Bit-Rate Pattern Coding with Recursive Orthogonal Decision of Parameters", by Shigeru Ono; U.S. Pat. No. 4,797,925, entitled "Method for Coding Speech at Low Bit Rates", by Daniel Lin; U.S. Pat. No. 4,815,134, entitled "Very Low Rate Speech Encoder and Decoder" by Joseph W. Picone et al.; U.S. Pat. No. 4,903,303, entitled "Multi-Pulse Type Encoder Having A Low Transmission Rate", by Tetsu Taguchi; and U.S. Pat. No. 4,932,061, entitled "Multi-Pulse Excitation Linear-Predictive Speech Coder", by Peter Kroon et al.
Other known voice encoding techniques are not degraded by white noise, but do have difficulty with coherent noise. One example of such a technique is known as Synchronized-Overlap-Add (SOLA). By example, U.S. Pat. No. 4,864,620, entitled "Method for Performing Time-Scale Modification of Speech Information or Speech Signals", by L. Bialik discloses a method for determining a value of an overlap and a windowing of the speech signal. However, it is believed that the presence of correlated noise will capture the overlap calculation and degrade the speech quality.
The present inventors describe an improved SOLA technique in an article entitled. "Some improvements on the synchronized-overlap-add method of time-domain modification for real-time speech compression and noise filtering", IEEE Journal on Acoust. Speech and signal Proc., Vol. 36, 1988, pp. 139-40.
One of the most severe environments for voice compression is in a vehicle where there exists both white noise, due to, for example, the wind, and coherent road noise and motor noise. Achievement of low bit rate voice encoding in these circumstances is difficult.
It is thus one object of this invention to provide a low bit rate voice encoding technique that provides intelligible speech at low signal-to-noise ratios.
It is a further object of this invention to improve the signal-to-noise ratio for low bit rate encoded speech, and to suppress both white noise and coherent noise when digitally encoding speech.